1. Field of the Invention
The present invention relates to an electronic device for use at high frequencies comprising a printed circuit board joined to a cavity resonance filter, and having the functions of a filter used in a miniaturized radio communications device.
2. Description of the Related Art
In recent years, there has been remarkable development of radio communications devices, such as cordless telephones and mobile telephones. The miniaturized shape and moderate cost of such radio communications devices have been the primary factors behind their proliferation.
As is well known, various types of functional circuits and devices, such as an antenna, a filter, and an amplifier, are required to form a radio communications device. The functional circuits are provided on a printed circuit board, and various other devices are mounted on the printed circuit board to form a radio communications device.
Several filters, such as an antenna filter, an intermediate-frequency filter, and the like, are required to form a communications device, though their number, function (LP, HP, etc.) or position and the like differ according to the communications method used. For example, in the case of an antenna bandpass filter connected to an antenna, in recent communications devices, an individual component which fulfils a predetermined function, such as a surface acoustic wave filter or a ceramic filter, are used as the bandpass filter. This is because a surface acoustic wave filter, a ceramic filter, and the like, have frequency characteristics which are superior to conventional LC filters, and also have a miniaturized component shape. Consequently, when a surface acoustic wave filter or a ceramic filter is used, the communications device can be miniaturized.
By using surface acoustic wave filter, a ceramic filter, or the like, it is possible to miniturize the communications device more than when a conventional LC filter is used. However, a surface acoustic wave filter, a ceramic filter, or the like, usually comprises an independent electronic component (termed a discrete component) functioning as a filter, which is mounted on the printed circuit board and electrically connected to a wiring pattern thereon.
Since each discrete component must be purchased separately, and since filter-type discrete components are especially expensive, the manufacturing cost of the communications device is high. Furthermore, when discrete components are used, the size of the discrete components, and spaces and the like provided for mounting or soldering them on the printed circuit board, increase the size of the communications device, and make it difficult for the communications device to be miniaturized and made thin.
It is an object of the present invention to provide an electronic device having the function of a filter which enables a communications device to be miniaturized and made thin at low cost.
The electronic device of the present invention comprises a printed circuit board having multiple electronic elements mounted thereon; a ground pattern provided on the printed circuit board; a pair of spiral conductive patterns provided adjacent to each other on the printed circuit board, one end of each pattern connecting to the ground pattern; and a cavity case electrically connected to the ground pattern, and covering the top faces of the pair of spiral conductive patterns.